encoding/json: tags can't have Unicode punctuation

[chipaca]

What version of Go are you using (go version)?

1.18.3

Does this issue reproduce with the latest release?

1.18.3 is the latest release at the time of writing

What operating system and processor architecture are you using (go env)?

go env Output

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/john/.cache/go-build"
GOENV="/home/john/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/john/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/john/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/john/sdk/go1.18.3"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/john/sdk/go1.18.3/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18.3"
GCCGO="/usr/bin/gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build1281614776=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Try to use a struct with a tag that has non-ASCII symbols in it, e.g.

type T struct {
	Foo31 string `json:"foo-3·1"`
}

https://go.dev/play/p/IwgJHZCedCE

What did you expect to see?

Tags can be anything, and JSON places no limits on this either (the name of a name/value pair is “a string”), and the encoding/json package mentions no limits, so I'd expect it to just work: the above playground link should produce

{"foo-3·1":"hi"}
<nil>

What did you see instead?

the tag is silently ignored and you get

{"Foo31":"hi"}
<nil>

this is because encoding/json's isValidTag only allows (some) ASCII symbols:

go/src/encoding/json/encode.go

Lines 972 to 987 in 95b68e1

	func isValidTag(s string) bool {
	if s == "" {
	return false
	}
	for _, c := range s {
	switch {
	case strings.ContainsRune("!#$%&()*+-./:;<=>?@[]^_{|}~ ", c):
	// Backslash and quote chars are reserved, but
	// otherwise any punctuation chars are allowed
	// in a tag name.
	case !unicode.IsLetter(c) && !unicode.IsDigit(c):
	return false
	}
	}
	return true
	}

[seankhliao]

related #39189

cc @mvdan @dsnet